Update README.md for Stack

[ashmeet-ashmeet]

What is the change?

Added Documentation for Stack. It consists of basic operations like insertion, deletion, displaying elements, checking for empty stack. Besides, there's implementation, using both linked list and array.

Related issue?

closes: #121

Checklist:

Before you create this PR, confirm all the requirements listed below by checking the checkboxes [x]:

• [x] I have followed the Code of Conduct.
• [x] I have checked there aren't other open pull requests for the same update/change.
• [x] I have you tested the code before submission.
• [x] I have commented my code, particularly in hard-to-understand areas.
• [x] My changes generates no new warnings.
• [x] I'm (HSOC) Hack-Club-Rait Summer of code '22 Contributor